26
Chapter 2
are usually expressed as moles per liter, which provides a unit
of concentration based upon the number of solute molecules
in solution, as described next.
The
molecular weight
of a molecule is equal to the
sum of the atomic weights of all the atoms in the molecule.
For example, glucose (C
6
H
12
O
6
) has a molecular weight of 180
[(6
×
12) + (12
×
1) + (6
×
16)] = 180. One
mole
(abbreviated
mol) of a compound is the amount of the compound in grams
equal to its molecular weight. A solution containing 180 g of
glucose (1 mol) in 1 L of solution is a 1 molar solution of glu-
cose (1 mol/L). If 90 g of glucose were dissolved in 1 L of
water, the solution would have a concentration of 0.5 mol/L.
Just as 1 gram atomic mass of any element contains the same
number of atoms, 1 mol (1 gram molecular mass) of any mol-
ecule will contain the same number of molecules—6
×
10
23
(Avogadro’s number). Thus, a 1 mol/L solution of glucose
contains the same number of solute molecules per liter as a
1 mol/L solution of any other substance.
The concentrations of solutes dissolved in the body ﬂ
uids
are much less than 1 mol/L. Many have concentrations in the
range of millimoles per liter (1 mmol/L = 0.001 mol/L), while
others are present in even smaller concentrations—micromoles
per liter (1
µ
mol/L = 0.000001 mol/L) or nanomoles per
liter (1 nmol/L = 0.000000001 mol/L). By convention, the
liter (L) term is sometimes dropped when referring to concen-
trations. Thus, a 1 mmol/L solution is often written as 1 mM
(the capital “M” stands for “molar,” and is deﬁ ned as mol/L).
Hydrogen Ions and Acidity
As mentioned earlier, a hydrogen atom has a single proton in
its nucleus orbited by a single electron. A hydrogen ion (H
+
),
formed by the loss of the electron, is thus a single free proton.
Hydrogen ions form when the proton of a hydrogen atom in a
molecule is released, leaving behind the hydrogen atom’s elec-
tron. Molecules that release protons (hydrogen ions) in solu-
tion are called
acids,
for example:
HCl
⎯→
H
+
+
Cl
hydrochloric acid
chloride
H
2
CO
3
34
H
+
+
HCO
3
carbonic acid
bicarbonate
OH
OH
A
A
CH
3
—C—COOH
34
H
+
+ CH
3
—C—COO
A
A
H
H
lactic acid
lactate
Conversely, any substance that can accept a hydrogen ion
(proton) is termed a
base.
In the reactions shown, bicarbonate
and lactate are bases because they can combine with hydro-
gen ions (note the two-way arrows in the two reactions). It
is important to distinguish between the un-ionized acid and
ionized base forms of these molecules. Also, note that by con-
vention, separate terms are used for the acid forms, lactic acid
and carbonic acid, and the bases derived from the acids, lac-
tate and bicarbonate. By combining with hydrogen ions, bases
lower the hydrogen ion concentration of a solution.
When hydrochloric acid is dissolved in water, 100 per-
cent of its atoms separate to form hydrogen and chloride ions,
and these ions do not recombine in solution (note the one-
way arrow in the preceding diagram). In the case of lactic
acid, however, only a fraction of the lactic acid molecules in
solution release hydrogen ions at any instant. Therefore, if a
1 mol/L solution of lactic acid is compared with a 1 mol/L
solution of hydrochloric acid, the hydrogen ion concentration
will be lower in the lactic acid solution than in the hydrochlo-
ric acid solution. Hydrochloric acid and other acids that are
100 percent ionized in solution are known as
strong acids,
whereas carbonic and lactic acids and other acids that do not
completely ionize in solution are
weak acids.
The same prin-
ciples apply to bases.
It is important to understand that the hydrogen ion
concentration of a solution refers only to the hydrogen ions
that are free in solution and not to those that may be bound,
for example, to amino groups (R—NH
3
+
). The
acidity
of
a solution thus refers to the
free
(unbound) hydrogen ion
concentration in the solution; the higher the hydrogen ion
concentration, the greater the acidity. The hydrogen ion
concentration is often expressed as the solution’s
pH,
which
is deﬁ ned as the negative logarithm to the base 10 of the
hydrogen ion concentration. The brackets around the sym-
bol for the hydrogen ion in the following formula indicate
concentration:
pH = –log [H
+
]
Thus, a solution with a hydrogen ion concentration of
10
–7
mol/L has a pH of 7, whereas a more acidic solution
with a higher H
+
concentration of 10
–6
mol/L has a lower
pH of 6. Note that as the acidity
increases,
the pH
decreases;
a change in pH from 7 to 6 represents a 10-fold increase in
the hydrogen ion concentration.
Pure water, due to the ionization of some of the mol-
ecules into H
+
and OH
, has a hydrogen ion concentration
of 10
–7
mol/L (pH = 7.0) and is termed a
neutral solution.
Alkaline solutions
have a lower hydrogen ion concentration
(a pH higher than 7.0), while those with a higher hydrogen ion
concentration (a pH lower than 7.0) are
acidic solutions.
The
extracellular ﬂ uid of the body has a hydrogen ion concentra-
×
10
–8
mol/L (pH = 7.4), with a homeostatic
range of about pH 7.35 to 7.45, and is thus slightly alkaline.
Most intracellular ﬂ
uids have a slightly higher hydrogen ion
concentration (pH 7.0 to 7.2) than extracellular ﬂ
uids.
As we saw earlier, the ionization of carboxyl and amino
groups involves the release and uptake, respectively, of hydro-
gen ions. These groups behave as weak acids and bases.
Changes in the acidity of solutions containing molecules with
carboxyl and amino groups alter the net electric charge on
these molecules by shifting the ionization reaction to the right
or left according to the general form:
R—COO
+ H
+
34
R—COOH